Orthogonal Frequency Division Multiplexing (OFDM) is a radio access technology, which has been selected for a number of wireless communications systems, such as, for example, 3GPP LTE (3rd Generation Partnership Project—Long Term Evolution), WiFi (Wireless Fidelity) and WiMax (Worldwide Interoperability for Microwave Access). In OFDM, a number of narrow-band symbols are transmitted in parallel (each symbol on a respective sub-carrier) in a frequency domain. The transmitter and receiver in an OFDM system efficiently convert the symbols in the frequency domain to and from the corresponding time-domain waveform using IFFT (Inverse fast Fourier transform) and FFT (fast Fourier transform) operations respectively. A collection of narrow-band symbols, together forming an output of an IFFT and/or an input to a FFT, is henceforth denoted an OFDM symbol. An OFDM symbol may appear both as a time-domain OFDM symbol and as a frequency-domain OFDM symbol.
Since each individual sub-carrier is narrow-banded in OFDM, the narrow-band symbols are relatively robust against frequency-selective fading, i.e. dispersion in the propagation channel due to multiple signal paths. For moderate channel dispersion the channel remains approximately flat over the narrow frequency band corresponding to each narrow-band symbol in isolation, even if fading dips are experienced over the overall signal frequency band of a frequency-domain OFDM symbol. That the channel remains approximately flat over the narrow frequency band corresponding to each narrow-band symbol effectively limits the inter-channel interference (ICI).
In order to improve robustness against channel dispersion further, a cyclic prefix (CP) is often used in OFDM systems. The CP is introduced in a time-domain OFDM symbol by pre-pending a copy of the last part of the time-domain OFDM symbol prior to transmission. The application of a CP eliminates the inter-symbol interference (ISI), i.e. the interference between OFDM symbols, altogether provided the channel dispersion (i.e. the delay spread) is less than the length of the CP.
However, when OFDM transmission is attempted over heavily dispersive channels, the resulting ISI may not be negligible despite the degree of robustness built into the OFDM scheme and even if a CP is used. This is because the ISI is not entirely eliminated if the channel dispersion is greater than the length of the CP.
In a practical OFDM system, the length of the CP is a compromise. A long CP is beneficial because it provides ISI-counteraction in scenarios with long delay spreads. However, a long CP is a waste of transmission recourses in typical scenarios (i.e. with moderate delay spread not requiring an overly long CP to accommodate the ISI) due to the copying of part of the OFDM symbol. This is to say that when the CP is not really needed, the transmission energy used for the CP does not directly improve the available data rates or coverage. On the other hand, while not wasting a lot of transmission resources, a short CP only provides ISI-accommodation in scenarios with a small delay spread. The CP is usually predefined in an OFDM system and is therefore typically designed for a typical delay spread case.
In many communication systems several cell sites cooperate in transmitting the same signal. In such systems, effective multipath channels with very large delay spreads may be experienced at a receiver. This may be the case even if each individual channel from a single cell site is quite compact. Examples of systems where several cell sites cooperate in this way are broadcast systems having SFN—(Single Frequency Network) or CoMP—(Coordinated MultiPoint transmission) type deployments.
Thus, there are practical propagation scenarios for OFDM systems where the delay spread may exceed the CP and introduce ISI. For transmission at high coding rates, even moderate ISI will be detrimental to the signal reception and limit the data rates experienced by a user. As touched upon above, one way to accommodate this problem would be to make the CP longer. But if not all users are likely to experience such large delay spreads, adjusting the CP would be a waste of system capacity. In the systems where several cell sites cooperate in transmitting the same signal, the delay spread experienced by a user depends on the user's location in relation to the cell sites. Therefore, an adjustment of the CP length may be mostly a waste of system capacity.
Therefore, there is a need for methods and arrangements that improve the quality of reception in heavy multipath (e.g. a delay spread longer than the CP) without relying on an extended CP.